1. Field of the Invention
The present invention relates to a wind turbine system, particularly to a technique for dealing with accidental drop of the system voltage in a power system.
2. Description of the Related Art
Recent wind turbine systems are required to feed electric power to the power grid with improved stability and higher reliability. One important requirement is to satisfy low-voltage ride through (LVRT) requirement which requires that a wind turbine power system must remain connected to the power grid when the system voltage suddenly drops resulting from an accident in the power grid. Most conventional wind turbine systems are designed so that controllers and other auxiliaries operate on electric power fed from the power grid; however, such-structured wind turbine systems can not continue to operate when the system voltage accidentally drops. One important issue is that the system voltage drop may excessively reduce the load of the generator, resulting in an excessive increase in the revolution speed of the turbine rotor. The excessive increase in the revolution speed is undesirable in itself in terms of safety. Additionally, the excessive increase in the revolution speed may undesirably result in that an excessively large voltage is applied to a voltage converter connected with the generator. In order to avoid these, the wind turbine system must be stopped and disconnected from the power grid.
One approach for satisfying the LVRT requirement is to use an uninterruptible power supply (UPS), as disclosed in U.S. Pat. No. 6,921,985. In the technique disclosed in this patent, an uninterruptible power supply starts to feed electric power to necessary instruments (such as, a power converter, a turbine controller, a blade pitch control system, and a crowbar circuit), when the system voltage drop is detected; the uninterruptible power supply does not feed electric power to unnecessary instruments. Feeding electric power to the blade pitch control system allows pitch angle control of the wind turbine blades, while feeding electric power to the crowbar circuits allows protection of the power converter.
One issue of using a UPS is that the UPS requires lots of maintenance. Batteries within an UPS suffer from deterioration caused by aging, and therefore the batteries needed to be exchanged in order to maintain the performance of the UPS. This undesirably increases the labor and cost needed for the maintenance of the wind turbine system.
U.S. Pat. No. 7,095,129 also depicts a fact that there is no power available for the load control system that provides asymmetric load control for a wind turbine. This document discloses a wind turbine having a plurality of generators, of which a first generator is configured to provide power to an electric grid and a second generator is configured to provide power to the wind turbine during times of grid loss. The disclosed wind turbine is configured to utilize power provided by the second generator to reduce loads on the wind turbine during times of grid loss.
Japanese Laid-Open Patent Application No. 2004-140971 discloses a mechanism for contact-free feeding of electric power to instruments provided on a rotating body rotating with a turbine rotor. The disclosed mechanism provides contact-free feeding of electric power by using a rotary transformer or an induction machine, reducing the necessity of maintenance. This patent application, however, does not disclose a technique for dealing with an accidental system voltage drop.
There is a need for a technique for providing a wind turbine system with the LVRT performance by using a mechanism that requires reduced maintenance.